THE KIDNEY 435 



lateral margins they do not present the typical stations which are 

 characteristic of the preceding portion. 



5. The Loop of Henle. As the descending limb enters the loop 

 of Henle, the tubule makes an abrupt turn and returns toward the cortex. 

 The location of the loop, being dependent upon the variable location in 

 the cortex of the renal corpuscle and the variable length of the thin seg- 

 ment, may be in any portion of the medulla except the extreme tip of the 

 pyramids; its most frequent site, however, is near the junction of the 

 boundary and papillary zones. 



The structure of the loop may be that of either the descending or the 

 ascending limb of the typical loop. It is also subject to great variations, 

 since the change in structure from the narrow to the broad type, though 

 it typically occurs just prior to the formation of the loop, is frequently 

 delayed until well into the ascending limb. As a rule, the change in 

 type occurs earlier when the loop lies in the boundary zone, and later 

 when it occurs nearer the apex of the renal pyramid ; the thick ascending 

 limbs do not occur in the papillary zone of the medulla. 



6. The Ascending Limb of Henle 's Loop (The Broad or Thick 

 Limb). This portion of the tubule returns through the boundary zone 

 of the medulla and enters a cortical ray, its course being parallel to 

 that of the descending limb. It then passes toward the surface of the 

 kidney, but finally leaves the ray and enters the labyrinth to reach that 

 renal corpuscle (close to the vas efferens) from which the uriniferous 

 tubule took origin, and in relation to which the tubule again acquires 

 a tortuous course (distal convoluted portion). Within the boundary 

 zone of the medulla this portion of the tubule is much broader than the 

 preceding division, but it becomes somewhat reduced in size in its course 

 through the cortical ray. 



The epithelium of the ascending limb is of a short cuboidal form. 

 Its cytoplasm resembles that of the lining epithelium of the convoluted 

 portion, although the nuclei in the tubule of Henle are rather more dis- 

 tinct. Basal striations are also less distinct than in the convoluted tu- 

 bule, the lateral serrations less deep, and the cell outlines sharper. The 

 cells of this portion frequently possess a slightly imbricated arrange- 

 ment. 



The recent careful comparative studies of mammalian kidneys by 

 Peter ("Untersuchungen iiber Bau and Entwickelung der Niere," Jena, 

 1909) have revealed certain important details. Peter divides the medullary 

 portion of the renal pyramid into an outer and inner zone; these portions 

 have no precise correspondence with the boundary and papillary zones above 



